Air Force pararescuemen, Arizona Parks department personnel and local fire and rescue crews participate in a mass causality exercise April 13 at the Grand Canyon. (Staff Sgt. Tim Chacon / Air Force)

They are the stuff of nightmares: a massive earthquake hitting Arizona with hundreds of casualties; U.S. civilians disappearing abroad; a U.S. Embassy stormed and taken by terrorists.

These are fictionalized scenarios Air Force rescue crews faced this month in the Arizona desert.

Exercise Angel Thunder kicked off April 8, made up of more than 4,000 personnel, including Air Force rescue crews, Marine Corps reconnaissance troops, Navy assets, and local and federal agencies. It’s the largest rescue training exercise in the world, and it was held among one of the grandest backdrops in the nation: the Grand Canyon.

“We go to combatant commanders and ask them what personnel recovery situations keep you up at night,” said Col. Jason Hanover, the 563rd Rescue Group Commander and Angel Thunder exercise director.

This year’s iteration includes three large-scale events.

The first was requested by U.S. Northern Command to focus on defense support to civil authorities: a massive simulated earthquake with its epicenter near the Grand Canyon, involving local officials and the Federal Emergency Management Agency. The crews developed a plan together and requested military help in finding 200 personnel.

The next two seem pulled from recent headlines or based off real-world instances, but officials say they were long in planning: terrorists or insurgents taking U.S. civilians and facilities at foreign locations, requested by the U.S. Central Command and U.S. Africa Command.

In the first of the two scenarios, interagency agents ran into issues between an airport and a U.S. Embassy and were taken into custody. The agencies asked the Defense Department for help, pushing Marine Corps recon members into the area to track and find the missing civilians. Over the course of days, the Marines and other military personnel located the civilians, but what to do with them after finding them required planning and protocol.

Hanover said a main challenge of the exercise is the overlap of multiple military and civilian agencies, along with representatives from allied countries, and how to create a clear chain of command to react to the scenario.

“It’s the perfect example of where Angel Thunder has to meld the operational and tactical level of war into one coherent scenario,” Hanover said.

The second of these scenarios includes the taking of an embassy by an armed force, which was planned before the Sept. 11, 2012, attack upon diplomatic facilities in Benghazi, Libya, that killed Ambassador Chris Stevens and three Americans.

That incident highlights the importance of rescue missions in the region, Hanover said. Like the airport mission, the embassy scenario will include Air Force and other military personnel, along with personnel for other agencies and representatives from other countries.

“Honestly, the real challenge is between the ears,” Hanover said. “It’s going to be thinking, problem-solving,” Hanover said.

The training is part of the spin-up process for 14 Air Force and Air National Guard Guardian Angel rescue teams, which brought their full crews and HH-60 Pave Hawks to the desert. Additional Air Force assets included HC-130P Combat Kings, MC-12 Liberty surveillance planes, an RC-135 Rivet Joint and multiple KC-135s. The Marine Corps sent recon and radio battalions, and the Navy’s 3rd Fleet off San Clemente Island, Calif., was involved.

Twelve partner nations are sending 144 personnel to train. With an additional nine countries observing, the exercise covered every area of responsibility except Africa, Hanover said.

Although Angel Thunder is now the largest exercise in the world, it began as an exercise for the Air Force combat search and rescue community. It then became the search-and-rescue exercise for Air Combat Command and then the Air Force itself. In 2006, it was certified by the Defense Department, and it will now be held every spring, said Brett Hartnett, the co-executive director and technical manager of the exercise, as well as a retired Air Force combat rescue pilot.

The $1.75 million exercise has survived while others face cutbacks, Hartnett said.